The invention relates to a process for calibrating the wheel speeds for a motor vehicle by calculating correction factors for the individual wheels in certain driving conditions.
Particularly in the case of motor vehicles with wheel-slip control systems, or in the case of those in which the driver is provided with information on the current dynamic state of the vehicle, there is a need for wheel speed calibration. Without such calibration, differences in the dynamic rolling radius of the tires of the different wheels--even if within permissible tolerances--can accidentally activate the wheel-slip control system or the driver information system. Such activation can be implemented by a reduction of the engine torque in the presence of excessive drive slip, increase of the engine torque in the event of excessive engine overrun torque, engagement of locks in longitudinal and transverse differentials, the admission of pressure to individual wheel brakes or to several at once to stabilize the vehicle or avoid a locking effect, and the activation of an optical, acoustic or haptic signal to inform the driver on the current dynamic state of the vehicle or the occurrence of a system intervention.
Wheel calibration can be employed, for example, in conjunction with anti-lock brake systems (ABS), anti-slip regulation systems (ASR) and electronic traction systems (ETS). In these cases, the risk that a wheel is about to lock up or spin is generally detected by the fact that the time rate of change of the measured speed of the wheel concerned is no longer within a specifiable normal range. (That is, when the wheel acceleration exceeds an adjustable threshold value, there is a risk that the wheel will lock up or spin.) In the text which follows, the term "acceleration" will be taken to mean both an actual positive acceleration and a negative acceleration, i.e., a deceleration.
In order to detect deviations from the normal, desired wheel-slip behavior as early and reliably as possible, high-accuracy control systems must take account of the fact that the speeds of the vehicle wheels are not equal even in the case of slip-free, purely rolling straight-ahead travel. Rather, differences in wheel speed can result, for example, from manufacturing tolerances in the production of the tires, variations in the degree of wear of the tires and similar factors. Thus, typical wheel speed differences of the order of few percent cause a deviation in the peripheral speeds of the wheels of about 1 km/h at a speed of travel of about 100 km/h, a value which must be taken into account in modern all-wheel and wheel-slip control systems. That is, wheel-speed is calibrated by processing measured wheel speeds, taking into account the differing rolling circumferences of the individual wheels. Such rolling circumferences, it should be noted, are subject to continuous changes with respect to time, before they are evaluated by the downstream control system, e.g. an all-wheel drive and/or wheel-slip control system.
German patent document DE 40 19 886 C1 describes a two-stage wheel-speed calibration procedure of this kind in which, in a first stage, the wheels are calibrated in pairs on the same side and, in a subsequent second stage, are calibrated with respect to a reference wheel. In this arrangement, the first stage is activated only below a first limiting speed, and the second stage is activated only above a second limiting speed. In either case, there must be sufficiently little cornering, for which purpose, this patent envisages, as an alternative, either checking whether, after differention with respect to time, the magnitudes of the wheel speed values for each wheel are below a specified limiting value or checking whether the magnitude of the difference between the two averages of wheel speeds on the same side falls below a specified limiting value.
Unpublished German Patent Application P 43 27 491.9 likewise discloses a two-stage wheel-speed calibration procedure, in which a rapid, coarse calibration with respect to a wheel chosen as a reference is carried out in a first stage, and a pairwise fine calibration is carried out in a second stage, either on the same side or on the same axle, depending on the existing driving torque.
German patent document DE-OS 41 30 370 A1 describes a single-stage wheel-speed calibration procedure of the generic type which detects cornering by checking whether the left/right speed deviation remains below a specified limiting value; or, as an alternative, cornering sensors are used.
The object of the present invention is to provide a wheel-speed calibration process of the type described at the outset which requires little outlay in terms of sensors, can be carried out as rapidly and reliably as possible and, in particular also, can be carried out during active intervention of a wheel-slip control system.
This and other objects and advantages are achieved according to the invention, in which a single iteration of the procedure can be carried out in a comparatively short time period. A particular advantage of the process according to the invention is that it can be carried out in a fault-free manner even during active phases of a wheel-slip control system in order to re-exit from the wheel-slip control without the wheel calibration inhibiting itself. According to the invention a special process is used to detect straight-ahead travel; in particular the variation with time of the left/right speed deviation of at least the driven axle is differentiated during a specified time period, and a limiting-value undershoot is detected. This procedure avoids the risk that the wheel calibration will even out the speed differences during an intended wheel-slip control phase. It also avoids a loss of stability in the absence of a system intervention, since, in the unstable driving state, the wheel speeds change continuously, which is reflected in a limiting-value undershoot of the left/right speed deviation differentiated with respect to time, after which the wheel calibration is inhibited. Since the driven wheels have the same left/right speed deviation during cornering as the wheels of the non-driven axle but, in addition, respond to travel over .mu.-split roadways, consideration of the driven wheels is, per se, sufficient for detection of cornering since the driven wheels represent the more stringent condition. This helps to minimize the effort involved in the procedure, and the run time for one cycle of the procedure.
It is, of course, also possible, as an alternative, to use the time derivatives of the left/right speed deviation of all the axles for the detection of cornering, and to infer that sufficiently little cornering exists only if all of these derivative values remain below a defined limiting value. The specification of a time period during which, for the detection of a sufficient degree of straight-ahead travel, the left/right speed deviations differentiated with respect to time must remain below the specified limiting value, prevents erroneous detections during travel around a uniform curve since, in such driving situations, the left/right speed deviation is briefly constant and, as a result, its time derivative is zero. Consequently, the time period provided for this purpose is set to a value which encompasses the time period for turning operations and cornering which occur during normal driving.
After an engine start, the process operates continuously. The correction factor is redetermined only when, in addition to sufficiently little cornering, the brakes are not actuated for the same time period, a certain minimum speed is exceeded and vehicle acceleration is sufficiently low. If the engine torque is available as information (e.g. by a databus), it is possible, as an alternative, instead of the vehicle acceleration value to use the driving torque as a corresponding decision criterion to check sufficiently low-slip travel, and it may thereby be possible to achieve higher calibration accuracy. The specification of a certain minimum speed serves to even out the Ackermann-related difference between the average front-axle speed and the average rear-axle speed by means of the effect of the attitude angle. In addition, the effects of measurement errors and deviations in the speed detection decrease with higher speed. The speed-correction factors determined by the procedure then permit the formation of corrected, mutually matched wheel speeds by multiplication of the respectively measured speed by the associated correction factor.
In another embodiment of the invention, the arithmetic mean of the filtered wheel-speed values of all wheels is chosen as the reference speed. Although, in contrast to conventional procedures (in which the reference speed is the speed of a selected wheel), this makes it necessary to calibrate all four vehicle wheels, the deviations of the respective wheel speeds with respect to the reference speed are comparatively low and balanced. As a result, the calibration process reaches the calibrated zero level relatively rapidly with stepwise alteration of the speed-correction factor; even, for example, after the fitting of a wheel which differs greatly from the others. Calibration with respect to the arithmetic mean also causes the average correction factor of all the individual correction factors to retain the value 1, and a gradual drift of the correction factors (for example with the correction factors in each case being stored after the ignition has been switched off) is thus avoided.
If, on the other hand, calibration is performed relative to a reference wheel, instead of the unvarying selection of the reference wheel as one wheel of a non-driven axle, it is possible to select in each case as the reference wheel that wheel whose speed deviates least from the arithmetic mean of all the filtered wheel speeds, so that the choice varies. It is then no longer necessary to calibrate with respect to the reference wheel.
In contrast to calibration with respect to a fixed specified reference wheel, calibration with respect to the arithmetic mean (or with respect to a reference wheel which in each case comes closest to it) eliminates the risk that two or three wheels rotating relatively equally will be calibrated with respect to a wheel which deviates greatly from them.
In a further embodiment of the invention, the adaptation of the correction factors to the respective newly measured wheel speeds takes place in steps, thus preventing undesirably severe brief fluctuations in the correction factors, e.g. due to effects of the roadway.
In still another embodiment, an offset factor is added to the wheel-speed correction factors of the driven wheels, which allows for the drive slip that occurs during constant travel on the flat. If there are no continuous data available on the engine torque, a fixed offset factor can be chosen. If, on the other hand, such data are available, the offset factor can be chosen as a function of the respective drive or engine torque. In this way, it is possible to allow for trips which are almost without vehicle acceleration, but do involve a high driving torque, (e.g., on long upward slopes, or with a trailer or with a high engine overrun torque on long downward slopes).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.